George Monbiot: repeated that he was 'extremely sorry' for the erroneous tweets. Photograph: Alicia Canter

The Guardian columnist George Monbiot has reached what he called an "unprecedented" libel settlement with Lord McAlpine, pledging to carry out three years of charity work as recompense for Twitter messages that wrongly linked the former Conservative chairman with an allegation of child sex abuse.

Monbiot repeated that he was "extremely sorry" for the erroneous tweets sent in November last year and said he would carry out work amounting to £25,000 on behalf of three charities.

The idea was suggested by McAlpine's lawyer, Andrew Reid of the law firm RMPI, as an opportunity to make some amends, Monbiot said in a statement released on Tuesday.

He added: "I accepted the suggestion immediately and without reservation. I feel the proposed settlement reflects very well on Lord McAlpine, who is seeking nothing for himself, but wants to see work done which could be of great benefit to others.

"I am currently working on finding three charities to whom I can be useful and with whom I can build a good relationship, with a particular emphasis on groups helping dispossessed or abused or destitute children, groups helping children to reconnect with nature and groups seeking the restoration of damaged ecosystems. I am also investigating creative and interesting ways of generating value or raising money for them.

"I have been prompted by Lord McAlpine's admirable proposal to do the right thing – something I should have been doing all along – and I hope very much that I am able to do it well."

The columnist and environmental campaigner had 55,000 followers on the social network when he contributed to the mass libel, which was sparked by pre-publicity for the BBC's calamitous Newsnight programme on 2 November.

On Tuesday, Monbiot repeated his apology for his "stupid and thoughtless" tweets. "I sent tweets hinting that Lord McAlpine was the person discussed in the Newsnight investigation of child abuse in North Wales," he said.

"As we now know, Newsnight's story was completely wrong. In doing so, I helped to stoke an atmosphere of innuendo around an innocent man ... I remain extremely sorry for the harm I helped to cause him."

Monbiot ended with a cautionary note for Twitter users: "Finally, please make sure you check your facts and think before you tweet."

In a statement, Lord McAlpine's legal representative said: "Lord McAlpine welcomes George Monbiot's statement and settlement relating to his defamatory tweets and is pleased to have reached a settlement whereby a number of charities will benefit from these defamatory actions.

"This is a ground-breaking settlement which underlines the danger of the misuse of the internet."

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